Question

Which of the following aldehyde does not give Cannizzaro reaction?

• A. HCHO
• B. \( \mathrm{CCl_3CHO} \)
• C. Benzaldehyde
• D. \( \mathrm{CH_3CH_2CHO} \)

Hint:

Cannizzaro reaction is given only by aldehydes without \( \alpha \)-hydrogen. If an aldehyde has \( \alpha \)-hydrogen, think of aldol reaction instead of Cannizzaro reaction.

Answer 1

The Correct Option is D

Step 1: Condition for Cannizzaro reaction.
Cannizzaro reaction is shown by those aldehydes which do not contain any \( \alpha \)-hydrogen atom. In the presence of concentrated base, such aldehydes undergo disproportionation to give one molecule of alcohol and one molecule of carboxylate salt.
Step 2: Check each option for presence of \( \alpha \)-hydrogen.

• (A) HCHO: Formaldehyde has no \( \alpha \)-carbon and therefore no \( \alpha \)-hydrogen. It gives Cannizzaro reaction.
• (B) \( \mathrm{CCl_3CHO} \): Chloral has no \( \alpha \)-hydrogen because the \( \alpha \)-carbon is attached to three chlorine atoms. It gives Cannizzaro reaction.
• (C) Benzaldehyde: The carbon next to the aldehyde group is part of the benzene ring, so there is no \( \alpha \)-hydrogen available. It gives Cannizzaro reaction.
• (D) \( \mathrm{CH_3CH_2CHO} \): Propanal contains \( \alpha \)-hydrogen atoms on the carbon adjacent to the \( \mathrm{-CHO} \) group. Therefore, it does not give Cannizzaro reaction.

Step 3: Reason for exception.
Aldehydes having \( \alpha \)-hydrogen usually undergo aldol reaction in basic medium instead of Cannizzaro reaction. Since propanal has \( \alpha \)-hydrogen, it does not undergo Cannizzaro reaction.
Step 4: Conclusion.
Hence, the aldehyde which does not give Cannizzaro reaction is \( \mathrm{CH_3CH_2CHO} \).
Final Answer:\( \mathrm{CH_3CH_2CHO} \).